1. Field of the Invention
This invention relates to a heater for heating objects having different sizes.
2. Description of the Related Art
There have been known electronic copying machines and facsimile machines (hereinafter referred to as "electrophotographic processing apparatuses") which are operated by an electrophotographic process.
During the electrophotographic process, toner is transferred onto a paper sheet to be transferred (hereinafter referred to as the "transfer paper sheet"). The toner transferred onto the transfer paper sheet is simultaneously heated to be fused and pressed thereagainst by means of a heater such as a heat roller so that the toner is fixed to the transfer paper sheet.
Recently, an electrophotographic processing apparatus which uses the following heater instead of the heat roller is known.
The heater used in such an electrophotographic processing apparatus comprises an elongated substrate made of alumina ceramics or the like and a heating section formed by an elongated film made of silver-palladium alloy and extending along the substrate. The ends of the elongated heating section are in the general vicinity of the respective ends of the substrate. Terminals, which are connected to a power source, are provided on the respective end portions of the heating section. Based on such a heater the present inventors developed an improved heater in which an alumina substrate has a heating section of silver-palladium alloy on it and an electrode section on each end. A lead or leads, made of the same material of that of the heating section, are branched from intermediate portions of the heating section and extend along the substrate. A further terminal is provided on the free end of each lead and is selectively connected to the power source.
The terminals of the heater are selectively rendered conductive so that the portion of the heater which has a length corresponding to the size of the transfer paper sheet is heated and the temperature of the portion of the heater which the transfer paper sheet does not contact is low. This process prevents the heating section from being overheated.
However, the temperature of the portions of the heating section which the leads are branched is lower than desired, because the heating section is broadened at the branched portions so that corresponding electric resistances are reduced, thereby, lowering the temperature of the portions. Furthermore, because the leads are made of good electric conductors, heat generated at the branched portions of the heating section is easily radiated therefrom. Further, still a high thermal conductivity of the terminals causes the heat generated in the heating section to be radiated from the terminals, lowering the temperature of the portions of the heating section at the vicinity of the terminals.
Accordingly, the conventional heater has the drawback of lowering the temperature of the heating section at the branched portions and at the leads.
Even if the temperature is unevenly distributed on the surface of the heating section, the toner is well fixed to the transfer paper sheet at the portion of the heating section which is heated to a high temperature. However, the toner is not well fixed to the transfer paper sheet at the portion of the heating section which is heated to a lower temperature. Some toner easily falls off from the portion of the sheet which is at a lower temperature, resulting in an uneven fixture of the toner onto the transfer paper sheet.